Displays exist where a fixed volume of water in a tube is stirred by a paddle-wheel at the bottom. A vortex shape will form and a partial air column (a free surface) will be drawn down from the top. Other fixed-volume displays exist where a pump is used to draw water out of the bottom of a tube and where it is reintroduced on the tangent, either at the top or bottom of the tube. These systems create a vortex but suffer from the disadvantage that, with a sufficient flowrate, the air column which is drawn down from the top surface of the water can achieve an uninterrupted path into the inlet of the pump. This air entrainment into the pump can cause cavitation and the air that passes through the pump is passed back to the tube with the water. This causes an emulsion of bubbles and cloudiness which detracts very much from the display that is achieved. Re-introducing the water at the top of the column reduces this air bubble problem, but requires a supply pipe up the side of the water feature which also detracts from the appearance of the display.